1. Field of the Invention
This invention relates generally to ink compositions and, more particularly, this invention relates to ink compositions useful in jet printing applications with plain paper, and methods of ink jet printing on plain paper.
2. Description of Related Art
Printing according to the "ink jet printing" principle, and apparatus for carrying out such printing operations, are well known. In general terms, a fluid ink is forced, under pressure and often at an elevated temperature, through a very small orifice in a printing head.
In so-called "continuous" jet printing operations, ink droplets are passed through a charging area wherein individual droplets receive an electrical charge in response to a signal. The droplets then pass through an electrical field, causing a varied deflection of the individual droplets dependent on the intensity of the charge and field. The droplets are produced continuously, and are directed to the substrate to be printed or. alternatively, to a by-pass gutter.
Due to the nature of the "continuous" jet printing process, inks used therein must be conductive so as to accept a charge. Such inks are generally water-based, as aqueous solutions or dispersions are easily rendered conductive. Water-based inks are generally characterized as having a low viscosity and high volatility. Both characteristics can be disadvantageous.
If the viscosity of a jet printing ink is excessively low, misting and the production of stray droplets can result, which in turn result in poor print quality. High volatility of a jet printing ink can result in clogging of jet nozzles due to evaporation.
Also, only a few water-based inks are capable of use on plain paper (i.e. a paper not bearing a pigment coating) and are capable of providing a high dye concentration on the surface of the paper so that it is possible to produce fine-sized droplets and to avoid build-up of dyestuff on the substrate surface while providing high color intensity.
So-called "drop on demand" (DOD) systems differ from continuous jet printing systems in that ink droplets are expelled from the nozzle of a printing head only when required during the printing process.
In a DOD printing system, ink is provided from a reservoir via a supply system to a nozzle/actuator system in the printing head. The actuators are generally of the piezoelectric or bubble type which upon actuation deliver a pressure pulse to the ink and thereby to force a very small and defined volume of the ink from a chamber defined by the actuator through a jet nozzle associated therewith from which the volume is ejected as a droplet.
For use in a DOD printing system, it is important for the ink to have a relatively low viscosity so as to minimise the power required to eject the droplets. It is also important that the size of any particles in the inks be sufficiently small to minimise the risk of blockage in the ink passageways of the nozzle/actuator system.
On the other hand, however, where the ink is intended for printing on a fibrous substrate such as paper, the deposits of colorant provided on the substrate by each droplet must be sufficiently large not to fall through the gaps between the fibres in the surface structure of the substrate.
Since inks used in DOD systems need not be conductive, they have generally been formulated using primarily non-aqueous solvents such as ethylene glycols, particularly diethylene glycols, which are characterized as having low volatility. As such, ethylene glycol and similar solvent-based inks do not evaporate quickly and, thus, exhibit reduced clogging problems.
Moreover, solvent-based inks exhibit desirable drying characteristics and are biologically stable while water-based inks generally tend to sorb air from the environment and are susceptible to biological fouling.
However, ethylene glycol and other non-aqueous solvents have a relatively high viscosity (which varies significantly with temperature) and thus require relatively more energy for production of droplets.
In some cases, it may be desirable to use a non-conductive ink (or one having extremely low conductivity) in DOD printers. For example, in DOD actuators wherein piezoelectric electrodes are disposed within an ink-containing chamber, the ink is preferably non-conductive in order to minimize or eliminate electrochemical effects.
Another class of inks used in jet printing are referred to in the art as "hot melt" inks and comprise wax or other thermoplastic materials such that the ink is solid or semisolid at ambient temperatures but fluid at elevated temperatures. The heated ink fluid solidifies when it comes in contact with a paper target.
Such inks generally tend to exhibit excellent dye stability. lightfastness, and no solvent volatility. However, these inks accommodate only relatively low dye concentration and therefore necessitate the production of relatively large drops in order to provide acceptable color intensity. Also, the thermoplastic component of such inks tends to form raised waxy deposits which are easily smeared or scratched off by rubbing.
Hot melt jet inks tend to be highly viscous and thus require high energy input in order to eject the drop from the printer nozzle. Also, since the entire ink must be rendered fluid by heating, high energy inputs and lengthy time periods are required to reach the printer operating temperature.